Method of making deep drawing stainless steel



United States Patent METHOD OF MAKING DEEP DRAWING STAINLESS STEELWailace B. Letiingwell, Sharpsville, Pa., and Carl W. Wcesner, Warren,Ohio, assignors to Sharon Steel Cor poration, Sharon, Pa., a corporationof Pennsylvania No Drawing. Application September 22, 1953, Serial No.381,744

5 Claims. (Cl. 148-l2) This invention relates to the preparation ofchromium alloy steels for deep drawing operations. More particularly, itpertains to the heat treatment of these steels to obtain improvedductility and a minimum tendency to forming a surface banded structure,sometimes called roping, during such operations.

In the fabrication of deep drawn articles from stainless steels, theaustenitic grades of 18-8 type, containing approximately 18% chromiumand 8% nickel, have generally been preferred because of their superiorcorrosion resistance, excellent drawing quality and bright finish. Theterm stainless steel is used not only to refer to the above mentioned18-8 type, but also to include straight chromium steels, such as A. I.S. 1. types 410, 430 and 442.

Due to Government restrictions on the use of nickel in recent years,'theuse of straight chromium stainless steels has been prevalentparticularly for civilian purposes. For many applications it has beenpossible to substitute straight chromium steels, such as 12% Cr (type410, containing about 12% Cr) or 17% Cr grades (e. g. type 430containing 14% to 18% Cr, the optimum being about 17% Cr; or type 440containing 18% to 23% Cr, in accordance with A. I. S. I. commercialdesignations). Type 442 is very similar in metallographic structure andphysical properties to type 430 but has slightly better corrosionresistance, and both are transforming, stainless steels. The mostpopular substitute for the 18-8 type is the type 430 straight chromiumsteel containing approximately 17%. chromium. its corrosion resistanceapproaches that of the 18-8 type, and in formations requiring relativelylight plastic deformations, it can be easily polished to a high luster.Moreover, improvements in die design and lubrication have made possiblethe use of 17% chromium alloy steel strip for deep drawn formationswhich were formerly regarded as too severe for such grades of steel.

In formations requiring relatively light plastic deformations, such asmoldings and ornamental trim, the 17% chromium strip .steel is verysatisfactory. However, in deep drawn parts which have been severelystretched or strained, this type of steel hasthe disadvantage of formingdirectional lines or bands, sometimes called roping or washboard. Thesedirectional lines are ridges on the surface of the metal and are due toa warpage of the metal during plastic flow in forming. They are veryunsightly and they are both difiicult and expensive to remove bypolishing. Accordingly, attempts have been made to increase theductility of the straight chromium stainless steel strip of the 17%chromium type and to minimize the efiect culties in sion resistant steelhaving increased elongation properties which in turn are conducive togreater deformations for a given operation.

Finally, it is an object of the present invention to solve existingproblems in the art, to eliminate prior art difiithe deep drawing ofstraight chrome stainless steels, to generally improve straight chromestainless steel cold Working procedures and products, and to obtain theforegoing advantages and desiderata in a simple and effective manner.

These and other objects and advantages apparent to those skilled in theart from the following description and claims, may be obtained, thestated results achieved, and the described ditficulties overcome by themethods, steps, operations and procedures which comprise the presentinvention, the nature of which is set forth in the following statement,a preferred procedure of v. hichillustrative of the best mode in'whichapplicants have contemplated applying the principles'is' set forth inthe following description, and which are particularly and distinctlypointed out and set forth in the appended claims forming part hereof.

The nature of the improvements in method of making deep drawing straightchrome stainless steel of the present invention may be stated in generalterms as comprising subjecting the steel, after hot rolling to stripform of the desired thickness and prior to annealing the same, to aphase redistribution heat treatment by passing the strip through andheating it in a continuous furnace at from 1700 F. to 21'00 F. longenough to promote complete heating of the strip and to effect a break upof the prismastic grain structure in the hot rooled strip resulting fromdirectional hot rolling of a m aterialstraight chrome stainless steel-inwhich two metallic phases coexist at hot rolling temperatures but not atroom temperature, said heating time being about one to ten minutes attemperature, and then air cooling the thus heated strip material.

The material treated by the discoveries of the invention is straightchromium stainless steel (types 430 and 442) which has a compositionwhereby the metal contains two phases in the temperature range where hotworking or roll-- ing operations take place; the metal can'be readily1600 F. and 2400" and gamma iron phases which are present insubstantially of directional lines or hands, the ultimate aim being toequal proportions in said temperature range.

For the purpose of the present invention the chemical analysis of thesteel used is as follows:

C I Mn Si Ni I Or I .25% max. 1.00% max. 1.00% max. .50% max. 14% to 23%In addition, traces of other elements, such as P, S, Cu and possiblysmall amounts of Ti, N, Mo, etc. may exist as incidental impurities. Atypical type 430 analysis is as follows:

When steel of this analysis cools,

F. to 2750 F. As the temperature Patented Oct. 1, 1957 it solidifieswith dendritic patternat a temperature of approximately 2600 j dropsbelow the."

solidification temperature the solid metal consists of crystals of deltairon and gamma iron. As the metal cools below about 1650" F.,substantially the delta iron phase retains its body-centered cubiccrystal structure which is the same as alpha iron at room temperature,whereas the gamma iron phase is' substantially transformed into aproduct consisting of carbides in a matrix of alpha iron.

Upon heating the ingots 'in'the soaking pits at temperatures approaching2300 F., the delta and gamma phases return. Subsequently, the ingots arerolled into blooms between the temperatures of 2000 F. and 2300 F., andthe gamma and delta'phases, which'exist substantially independently ofeachother, are rolled out longitudinally 'in the direction of rolling byvirtue of the directional stresses imparted by the hot workingoperation. Normally, the blooms are then cooled to room temperature forthe removal of surface defects by usual surface conditioning operations;and when the blooms are so cooled, the delta and gamma phases transformin the manner previously described. However, when the blooms are againreheated for rolling into slabs, the delta and gamma phases againreappear in their directional alignment. If the blooms do not requiresurface conditioning prior to slab rolling, the blooms may be rolleddirectly to slabs without cooling, surface conditioning and reheating.This, however, is rule because seldom' is such as not to requireconditioning.

The rolling temperature in rolling blooms to slabs is usually in therange from about 2300 F. to 1600 F., and further directionalcharacteristics are imparted to the the exception and not the metalduring hot working in rolling the slabs. When the slabs are cooled, forsurface conditioning as by grinding and the like, the delta and gammaphases again undergo transformation as previously described. However,when the slabs are again reheated for hot rolling into strip form, thedelta and gamma phases again reappear.

During the hot rolling of the slabs to form hot rolled strip, the metalin the slabs, which maybe from 2" to 3 thick, is rolled out to athickness of .100 to .250 thick, again in a' temperature strip has aduplex structure consisting of low carbon ferrite, originatingsubstantially from the delta iron phase; and a decomposition product,possibly containing pearlite or martensite depending upon the rate ofcooling,

this constituent having been transformed from the gamma iron phase. Thisduplex structure has the arrangement of a cluster of small hands runningin the direction of rolling; while at right angles to the direction ofrolling there is no banding or directional structure. A microscopicspecimen out along any plane parallel to the rolling direction (such asa parallel to the surface) shows the directional structure. Theforegoingconstitute the major operations normally carried out inproducing hotrolled stainless steel strip.

Where cold rolled strip is produced the usual operations include thefollowing:

. Hot roll at from 2300 F. to 1600 F.

. Box anneal between-l400 F. and l600-F.

. Pickle to remove scale.

. Cold roll to some lighter gauge.

. Continuous anneal between 1350 Pickle to remove scale.

. Pinch pass if desired for F. and 1500 F.

very light reduction.

Where the operating conditions or the dimensions of V the final productrequire, steps'4,

5 and 6 may be repeated in that order after operation 6.

In the above procedure, step 1' has been described and the surfacecondition of a bloom range of from about 2300 F; down to 1600 F., inwhich temperature range the delta strip are made alongplane parallel tothe mill edge, or.

from hot or directional rolled strip. Then if rolled sheet product isbox annealed, pickled, cold rolled 4 includes those hot rollingoperations that are necessary to reduce the metal to usual hot rolledstrip thicknesses ranging from .100 to .250". At this stage, the metalis in the form of long coils which should be annealed. For this purposeit is preferred that the coils be box annealed by placing the same inasuitable closed container with or without packing material in order tominimize oxidation at a temperature rangeof 1400 to 1600 F. Aftercooling the metal is subjected to pickling to remove scale. It is thencold rolled to a lighter gauge, annealed at a temperature between 1350and 1500 F. and pickled again to remove scale. This may be followed by asocalled pinch pass through cold rolls in order to impart .a cold rolledsurface.

If the cold rolling and annealing operations are propcrly carried outand controlled, the finished cold rolled strip product has no apparentvisible directional properties until it is stretched or drawn. If,however, a sample of the cold rolled strip is heated somewhat above1650" F. and slow cooled, microscopic examination reveals a duplexstructure which has a directional orientation corresponding to that inthe hot rolled strip, indicating that the directional properties inducedby hot rolling are still inherent in and present in'thecold rolledproduct despite the various processing operations performed in reducingthe hot rolledstrip to finished cold rolled strip. If the finishedproduct in the annealed or soft state is severely stretched, as in atensile test, or formed into deep drawn parts, such as pans, or cups,directional lines of the character described develop and appear on thesurface of the stretched or deep drawn product.

We have established from another standpoint that thesedir'ectional linesare directly related to the duplex hot rolled structure imparted bydirectional hot working, by determining that the condition can bealtered by changing the direction of hot rolling at some appropriatestage of hot working. For example, if an ingot'is rolled into a sheetbar, say /2" thick, and short lengths of the sheet bar are cross rolledto form'.060 thick sheets, hot rolling in the sheet mill being done atright angles to the direction of the prior hot working operations, theproduct has distinctly difl'ferent properties from material produced thecross to say .020" gauge sheets, annealed and electro-pickled,stretching or cold drawing directional lines or roping characteristicsof material produced from strip hot worked entirely in one direction. i

Instead of roping in cross rolled sheets, a pebbled effect may appear orbe visible on the surface of a pan or cup deep drawn therefrom. Theseresults establish that the cold rolling does not, produce thedirectional lines. Microscopic samples of cold rolled material producedfromcross rolled sheet, heatedabove 1650 F.,. slow cooled, and examinedfor structure, disclose that the directional pattern differs frommaterial originating from hot rolled strip. The latter, shows a bandeclstructure in any direction parallel to the hot rblling direction; whiletransverse to the rolling direction there is no banding. In contrast,material originating from cross rolled sheet discloses no banding in aplane parallel to the surface,

' but discloses a checkerboard pattern. There is banding,

however, in planes perpendicular to the surface.

These investigations indicate that the directional orientation producedby hot rolling to sheet bar in one direction, followed by cross rollingat right angles in the sheet mill, persists after subsequent coldrolling and annealing operations. In material produced from cross rolledsheet, the checker-board, hot rolled structure seems to be related tothe pebbled surface obtained after stretching or drawing the finishedproduct, just asthe'hot rolled banding is related to the directionallines or roping on materialpro duced from hot rolled strip.

. Although cross rolling in a sheet millis a partial solu tion totheproblem', it is less economical to produce large does not tend toform the quantities of short sheetsthan long coils and it is moredifiicult to produce a cold rolled surface in sheets. Suflice it to say,it is more desirable to produce strip in large coils.

We have found it possible to produce a strip product having a minimum ofdirectional tendency and also better ductility by subjecting hot rolledstrip to a new and special heat treating procedure after hot rolling andprior to box annealing the same.

Accordingly, the hot strip is heated in a continuous furnace to atemperature substantially above the finishing temperature on the hotmill for a a partial breakdown of the directional banding. Thistemperature range is from 1700 to 2100 F., for a sufiicient time topermit thorough heating. The optimum temperature range is 1900 F. to2000 F., the preferred temperature being 1950 F. The time required isone to ten minutes at temperature depending on the strip mass orthickness. Lower temperatures are not effective, while highertemperatures may cause undesirable grain growth.

This heat treating operation is not a normalizing operation inasmuch asthe temperature must be substantially above that required for a phasechange, nor is it an annealing or softening operation for the reasonthat no softening or annealing occurs. In fact, the hardness may beslightly increased sometimes from 94 to 98 RB, Rockwell hardness. Ratherthe operation is a partial homogenizing procedure, the purpose of whichis to promote diffusion between the gamma and delta phases in atemperature range where they coexist and in the absence of mechanicalstress.

As a result, the banded structure is partially broken up and the grains,which are approximately polygonal prisms after hot rolling, areconverted to irregular polyhedrons of many faces. Thus there is apartial breakup of banding which greatly facilitates softening andequiaxing of the grain structure in subsequent operations. After thisheat treating operation the strip is air cooled.

The process of manufacturing the new deep drawing 17% chromium strip inaccordance with the invention and beginning with a slab is as follows:

1. Hot roll at from 2300 F. to 1600 F.

2. Phase redistribution heat treatment between 1700 F. and 2100 F.(preferably 1950 F.).

. Air cool to room temperature.

. Box anneal between 1400 F. and 1650" F.

. Pickle to remove scale.

. Cold roll to form lighter gauge.

. Continuous anneal between 1350 F. and 1500 F. Pickle to remove scale.

Repeat steps 6, 7 and 8, after step 8, if desirable, or pinch pass.

The phase redistribution heat treatment is, of course, the crucialfeature of this invention. Inasmuch as it is neither a softening nor ahardening operation, but is concerned with the arrangement of twometallic phases which coexist at hot rolling temperatures but not atroom temperatures, this heat treatment is believed to be unique inmetallurgical practices. This is believed to be particularly true owingto the position in which the operation is placed in the sequence ofoperations.

By carrying out the new procedure of the present invention, a new deepdrawing grade of chromium alloy stainless steel, such as 17% chromestainless steel, may be made, the phase redistribution heat treatmentbeing performed on the hot rolled strip after hot rolling and before boxannealing. The allowable range of heating is 400 F. between from 1700 F.to 2100 F., preferably 1950 F. The time-temperature cycle must besufiicient to effect a redistribution and reshaping of the grains,converting them from polygonal prisms to irregular polyhedrons andeffecting a partial breakup of the hot rolled banded structure.

In the metallurgical control of the phase redistribution heat treatment,the usual hardness and ductility tests are time suflicient to eifect ofno value and no testing method seems adequate excepting microscopicexamination, the effectiveness of the phase redistribution heattreatment being determined by metallurgical examination of microscopicsamples.

Even microscopic examination has its limitations. For instance, if type430 stainless steel is subjected to the phase redistribution heattreatment of the present invention, but the cooling is at a rapid rate,although the benefits of diminished roping are obtained, yet microscopicexamination does not disclose the irregular polyhedrons with the grainsof bainite approaching approximately spherical shape, even though theymay be present. Instead, .with rapid cooling the phase changesoriginating from thedelta and gamma iron phases both show a needlelikepattern of martensitic structure.

However, if the fast cooled material is reheated to say 900 F. and thencooled, that is to say if the material is drawn by reheating andcooling, the grain structure becomes more evident.

In other words, even though the breakup of the banded structure occursfrom heat treatment of the material in accordance with the invention, itmay possibly not be possible to see the effectiveness thereof bymicroscopic examination if the material has been rapidly cooled.

Cold rolled straight chrome stainless steel produced in accordance withthe procedure of the present invention has many improved properties. Ithas a more homogeneous microstructure; microscopic tests made aftervarious annealing operations disclosing a much better equiaxedmicrostructure than is common to usual straight chrome stainless steelstrip. Ductility of the finished product is much improved, showing anincrease of about 20% in the values obtained from usual straight chromestainless steel strip. For example, an average of about 25% elongationin 2" is obtained in conventional type 430 stainless steel while anelongation of about 30% in 2" is obtained in steel products made inaccordance with flie invention.

The tendency toward directional lines in the finished product .isgreatly reduced. Thus when the finished product made in accordance withthe present invention is severely stretched or deep drawn, it forms aminimum of directional lines or roping. After deep drawing, the materialis much easier to polish to a high luster, because of the diminishedroping effect, so that polishing costs are greatly reduced andappearance of the finished deep drawn part is improved.

Accordingly, the present invention provides a new procedure for makingcold rolled straight chrome stainless steel strip; provides improvedcharacteristics or properties in the finished objectives, advantages anddesiderata in a simple and etfective manner; and overcomes prior artdifficulties and solveslong standing problems in the art.

In the foregoing description, certain terms have been used for brevity,clearness and understanding; but no unnecessary limitations are to beimplied therefrom beyond the requirements of the prior art, because suchterms are utilized for descriptive purposes herein and not for thepurpose of limitation and are intended to be broadly construed.

Moreover, the description of the improvements or discoveries is by wayof example and the scope of the present invention is not limited to theexact details described provided that the new procedure is carried outat the time and in the manner described in the manufacture of straightchrome stainless steel.

Having now described the features, discoveries and principles of theinvention, the procedures of preferred method steps thereof, thecharacteristics of the new products obtained thereby, and theadvantageous, new

and useful results provided; the new and useful methods,

steps, operations, procedures, discoveries and principles, 1

and mechanical equivalents obvious to those skilled in the art, are setforth in the appended claims.

product; satisfies the foregoing- 1. The method of making deep drawingquality, cold rolled, 14%to 23% straight chromium, transforming,stainless steel strip capable of being deep drawn with a minimum ofdirectional lines imparted by directional hot rolling of the deltaandgamma iron phases which exist at hot rolling temperatures but not atroom temperature in 14% to 23% straight chromium transforming stainlesssteel, and which delta and gamma iron phases transform upon cooling to aduplex structure consisting of ferrite substantially originating fromthe delta iron phase and a decomposition product originating from thegamma iron phase, arranged in a cluster of small bands running in thedirection of rolling; the steps of hot rolling slabs of 14% to 23%straight chromium, transforming, stainless steel at hot rollingtemperatures of from about 2300 F. to 1600 F. in one direction to' formhot rolled strip; continuously heating the hot rolled strip to atemperature in the range of from 1700 F. to 2100" F. for from one to tenminutes to promote a redistribution between the gamma and delta phasesin a temperature range where they coexist and in the absence ofmechanical stress, to efiect a breakup of the banded structure and toredistribute, reshape and convert the grains which are approximatelypolygonal prisms after hot rolling into irregular polyhedrons of manyfaces tending to approach We claim:

va spherical shape upon cooling; then annealing the strip at from 1400F. to 1650 F., then cold rolling the strip to desired gauge, and'thencontinuously annealing the cold rolled strip at a temperature between1350" F. and 1500 F. to-provide annealed cold rolled stainless steelstrip material having elongation of, about 30% in two inches.

2., The method of makingdeep drawing quality, cold rolled, 14% to 23%straight chromium, transforming,

stainless steel strip capable of being deep drawn with minimum ropingresulting 'from banding produced by longitudinal hot rolling directionalstresses imparted. to the delta and gamma iron phases which exist at hotrolling temperatures but not at room temperature in 14% to 23% straightchromium transforming stainless steel; the steps of hot rolling slabs of14% to 23% straight chromium, transforming, stainless steel at.hotrolling temperatures of from about 2300 F. to 1600 F. in onedirection to form hot rolled strip, continuously heating the hottrolledstrip to a temperature in the range of from 1700 F.. to 2100? F. forfrom one to ten minutes to effect a breakup of directional bandingpresent in the t directionallyhot rolled strip as a result of hotrolling the delta and gamma iron phases at hot rolling temperatures,then annealing the strip at from .1400 F. to 1650 F., and then coldrolling the strip to desired gauge, and then continuously annealing thecold rolled strip at a tempera-- ture between 1350 F. and 1500 F.

3. The method set forth in claim 2 in which the hot rolled strip iscontinuously heated to a temperature between 1900 F. and 2000 R, and inwhich the strip is aircooled after. such heating.

4. The method set forth in claim 2 in which the hot.

rolled strip is continuously heated to a temperature of 1950 F. i r r 5.The method of making, cold rolled, 17% straight chromium, transforming,stainless steel strip having a minimum of directional lines whenseverely stretched or deep drawn, including the steps of hot rollingslabs of 17% straight chromium, transforming, stainless steel strip inone direction at hot rolling temperatures of from about 2300" F. to 1600F. where delta and gamma iron phases coexist to form hot rolled strip,continuously heating the hot rolled strip to a temperature in the range'of from 1700 F. to 2100" F. for from one to ten minutes to efiectabreakupof directional banding present in the directionally hot rolledstrip as a result of hot rolling the delta and gamma iron phases at hotrolling temperatures, then annealing the strip at from 1400 F. to 1650F., then cold rolling the strip to desired gauge, and then continuouslyannealing the cold rolled strip at a temperature between 1350 F. and1500 F.

7 References Cited in the tile of this patent UNITED STATES PATENTSCarruthers Mar. 3, 1954 Kiefer et al. Dec. 4, 1956 OTHER REFERENCES

1. THE METHOD OF MAKING DEEP DRAWING QUALITY COLD ROLLED, 14, TO 23STAIGHT CHLOMIUM TRANSFORMING, STAINLES STEEL STRIP CAPABLE OF BEINGDEEP DRAWN WITH A MINIMUM OF DERECTIONAL LINEA IMPARTED BE DIRECTIONALHOT ROLLING OF THE DALTA AND GAMMAN IRON PHASES WHICH EXIST AT HOTROLLING TEMPERATURE BUT NOT AT ROOM TEMPERATURE IN 14% TO 23% STRAIGHTCHROMIUM TRANSFORMING STAINLESS STEEL STRIP CAPABLE OF BEING DEEP DRAWNWITH A TRANSFORM UPON COOLING TO A DULPEX STRUCTURE CONSISTING OFFERRITE SUBSTANTIALLY ORIGINATING FROM THE DELTA IRON PHASSE AND ADECOMPOSITION PRODUCT ORIGINATING FROM THE GAMMA IRON PHASE ARRANGED INA CLUSTER OF SMALL BANDS RUNNING IN THE DIRECTION OF ROLLING THE STEPSOF HOT ROLLING SLABS OF 14% TO 23% STRAIGHR CHLOMIUM, TRANSFORMING,STAINLESS STEEL AT HOT ROLLING TEMPERATURE OF FROM ABOUT 2300* F. TO1600* F. IN ONE DIRECTION TO FROM HOT ROLLED STRIP; CONTINUOUSLY HEATINGTHE HOT ROLLED STRIP TO A TEMPERATURE IN THR RANGE OF FROM 1700* F. TO2100* F. FOR FROM ONE TO TEN MINUTE TO PROMOTE A REDISTRINUTION NETWEENTHE GAMMA AND DELTA PHASE IN A TEMPERATURE RANGE WHERE THEY COEXIS ANDIN THE ABSENCE OF MECHANCAL STRESS, TO EFFECT A BREAKUP OF THE BANDEDSTRUCTURE AND TO REDISTRIBUTE, RESPHE AND CONVERT THE GRAINS WHICH AREAPPROXIMATELY POLYGONAL PRIMS AFTER A HOT ROLLING INTO IRREGULARPOLYHEDDRONS OF MANY FACES TENDING TO APPROACH A SPHERICAL SHAPE UPONCOOLING; THEN ANNEALING THE STRIP AT FROM 1400*F. TO 1650*F., THEN COLDROLLING THE STRIP TO DESIRE GAUGE, AND THEN CONTINUOUSLY ANNEALING THECOLD ROOLED STRIP AT A TEMPERATUR BETWEEN 1350* F. AND 1500* F. TOPROVIDE ANNEALED COLD ROOLED STAINLESS STEEL STRIP METERIAL HAVINGELONGATION OF ABOUT 30% IN TWO INCHES.